Try clearing the web content cache and disabling Firefox from using any proxy server. Here's how:

(1) Clear Firefox's Cache

See: How to clear the Firefox cache (only select Cached web content, don't clear all cookies and site data)

If you have a large hard drive, this might take a few minutes.

(2) Disable using any system-level proxy

Open the Settings page using either:

• "3-bar" menu button (or Tools menu) > Settings (previously "Options")
• type or paste about:preferences into the address bar and press Enter to load it

In the very tiny search box at the top of the page, type proxy and Firefox should filter to the "Network Settings" section.

Click the Settings button, select "No Proxy" near the top of the dialog, and click OK near the bottom of the dialog.

Any difference?

You can try to disable DNS over HTTPS.

• https://support.mozilla.org/en-US/kb/firefox-dns-over-https
• https://support.mozilla.org/en-US/kb/dns-over-https-doh-faqs

Are you using a proxy or VPN?

You can check the connection settings.

• Settings -> General -> Network: Connection -> Settings

If you do not need to use a proxy to connect to internet then try to select "No Proxy" if "Use the system proxy settings" or one of the others do not work properly.

See "Firefox connection settings":

• https://support.mozilla.org/en-US/kb/websites-dont-load-troubleshoot-and-fix-errors
• https://support.mozilla.org/en-US/kb/firefox-cant-load-websites-other-browsers-can
• https://support.mozilla.org/en-US/kb/firefox-and-other-browsers-cant-load-websites

See also:

• https://appuals.com/pr-end-of-file-error/

Nothing worked. Also tried a lot of webpages like this one https://www.techinpost.com/how-fix-solve-secure-connection-failed-firefox-browser-error-code/ ...the problem is still there.

Is it always the same code: "PR_END_OF_FILE_ERROR" ?

Any difference if you use a full URL such as

• https://mail.google.com/mail/u/0/#inbox
• https://www.google.com/search?q=firefox
• https://www.youtube.com/@Mozilla

What kind of internet connection -- home service provider, work provider, public hotspot, VPN, something else?

Only "google owned web suite" sites are affected. Whole Internet is working great as expected... Im not a fan of the engine, but im using gmail and cant ignore youtube. And yes, is the same error all the time. And this bug have already 4 years and... is annoying.

and in this error window we dont have the option "Are you sure you want go to a insecure site? Yes/ No"

Some users have been experiencing a mid-session inability to connect to Google sites. That manifests differently than your situation because there is no error page, the connection just doesn't work. But you could try this change and see whether it makes any difference: turn off the "zero round trip resumption" feature:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

More info on about:config: Configuration Editor for Firefox. The moderators would like us to remind you that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future.

(2) In the search box in the page, type or paste security.tls.enable_0rtt_data and pause while the list is filtered

(3) Double-click the preference to switch the value from true to false

After that, try Google again in a new tab. Any improvement?

Also, just to confirm, when this problem hits in Firefox, can you visit the sites normally in Microsoft Edge or other alternate browser?

Now google sites look ok :). No connection errors :) Thanks you very much guys!! At least from now all is looking fine! Big Thanks!